So all Sensiron sensors are tuned for minimum 400ppm, other than this one: https://sensirion.com/products/catalog/STC31 https://sensirion.com/products/catalog/

So all Sensiron sensors are tuned for minimum 400ppm, other than this one: https://sensirion.com/products/catalog/STC31 https://sensirion.com/products/catalog/STC31 The problem is that it gives you percentage of CO2 in air instead of ppm, it can go all the way down to 0% though. I guess there must be a way to figure out the ppm based on that? I guess? Here's an example of a product with this sensor already integrated: https://www.sparkfun.com/products/18385 https://www.sparkfun.com/products/18385 Also I want to point out that their other sensors(SCD40 and SCD41) will actually give you a value even below 400ppm - just that their accuracy drops below that value. So it might say it's 300ppm when in reality it's 250ppm. Not sure how important it is for you.